Another surprise was that the two Vibrio species use different molecules for signaling and different mechanisms for detecting those signals. Because they are so closely related, Bassler and everyone else had guessed they would use the same signaling machinery. It was interesting work but still considered at that time to be an anomaly of these two bizarre bacteria.

"Everybody kept asking me 'Why do you want to study bioluminescence?' and I kept trying to explain that I wasn't studying bioluminescence," says Bassler. "I was trying to study cell-to-cell communication. Bioluminescence was the thing we could see in the lab—a remarkably easy way to detect when cells 'talked to each other' with the aid of these chemical words."

In the early 1990s, other scientists began to investigate chemical signaling in terrestrial bacteria as well, finding that it was not limited to marine oddities. "Now we understand that probably all bacteria use chemical communication, likely with multiple signals," Bassler says. "They have incredibly complicated chemical lives, of which we so far understand almost nothing."

Bacterial chemical communication is now referred to as "quorum sensing." That is, the bacteria determine when their population has attained a quorum, so to speak, and that tells them they can go into action in unison—doing whatever they need to do. Not only do they converse with one another and then act simultaneously, they also divide up chores and specialize.

"They're recognizing that if they have the right number—and they synchronize their behavior—they can carry out tasks that they could never accomplish if they acted as individuals," explains Bassler. Thus, long before the appearance of multicellular organisms, bacteria had devised a way to act together, as if they were a multicellular organism.

Bassler's work gave bacteria a central role in the development of higher forms of life on earth. They have survived for billions of years not only because they are tough but also because they are far more sophisticated than anyone had realized.

These big-picture observations attracted other scientists' attention. Princeton professor Thomas J. Silhavy, who helped bring Bassler to Princeton, says he saw that her work on Vibrio "could lead to some very interesting biology." His instinct was right. "She is the leader in quorum sensing," he says. "Which is a pretty big deal." Yet again it wasn't just Bassler's research that attracted the attention. "We were very, very impressed with her obvious enthusiasm," Silhavy says.

		View image I Am One of You. Illustration by Charmaine Wheatley Script by Carol Cruzan Morton

That second chemical signal she had discovered in V. harveyi revealed what Bassler calls a bacterial Esperanto—a universal language that bacteria use to talk to other species. The first signal, called AI-1 (AI stands for autoinducer, because it induces the bacteria to act), was unique to V. harveyi; it didn't exist in any other species of bacteria. But Bassler found that all kinds of bacteria produced the second molecule, AI-2, suggesting that it had an entirely different role. While the bacteria used AI-1 to talk among themselves, AI-2 was a common language among different species. And, critically, it helps them distinguish between like and unlike bacteria—self and other.

"This is the basis for cells specializing," Bassler explains. "If you have a mix of species, different groups can do different things." Bacteria, in other words, not only invented multicellularity, they also invented the kind of division of labor seen in multicellular organisms.

For example, biofilms—like the sticky glaze that collects on teeth overnight—are composed of hundreds of species of bacteria, each performing a specialized job to keep the "organism" alive. "You brush them off, and then the next morning they come back and they're in exactly the same organization," Bassler says. "Only a couple of species are the primary colonizers, then the next guys depend on them to stick." Others provide nutrients, and so on. All in all, it's a mutually advantageous architecture that allows all the species to flourish.

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